Docker mode pre-commit can't use JAVA_HOME defined in dockerfile

Over in HBase we use different JDKs depending on the branch, due to our compatibility promises in major release lines. For test-patch runs, this changes the set of JDKs we install in each branch's Dockerfile.

While recently trying to chase down why we were getting JDK8 used to test a branch that's JDK7+, I realized that the docker bootstrapping always overwrites the JAVA_HOME in the image with JAVA_HOME on the host. This includes when the host doesn't define a JAVA_HOME, effectively preventing a JAVA_HOME set by the image (either via ENV or by installing some package that sets it).

Workaround:

You can prevent this by replacing the docker_do_env_adds function in your personality. e.g. this version will only set JAVA_HOME in the docker instance if the host defines it. We then make sure all of our uses of test-patch unset JAVA_HOME before invocation.

# work around yetus overwriting JAVA_HOME from our docker image
function docker_do_env_adds
{
  declare k

  for k in "${DOCKER_EXTRAENVS[@]}"; do
    if [[ "JAVA_HOME" == "${k}" ]]; then
      if [ -n "${JAVA_HOME}" ]; then
        DOCKER_EXTRAARGS+=("--env=JAVA_HOME=${JAVA_HOME}")
      fi
    else
      DOCKER_EXTRAARGS+=("--env=${k}=${!k}")
    fi
  done
}